Crestfallen heir ends date explosively (12)
I believe the answer is:
disheartened
'crestfallen' is the definition.
(I've seen this before)
'heir ends date explosively' is the wordplay.
'explosively' indicates an anagram.
'heir'+'ends'+'date'='heirendsdate'
'heirendsdate' with letters rearranged gives 'DISHEARTENED'.
(Other definitions for disheartened that I've seen before include "Upset; downcast" , "Having lost determination or confidence" , "Crestfallen" , "Discouraged" , "Despondent" .)